A gas turbine engine of the turbofan type has a fan section, a crxnpression section, a combustion section and a turbine section. An annular flow path for working medium gases extends axially through these sections. As the working medium gases are flowed along the flow path, the gases are pressurized in the fan and the compression sections and burned with fuel in the combustion section to add energy to the gases. The hot, high pressure gases are expanded through the turbine section to produce useful work for pressurizing the gases in the fan and compression sections and thrust for propelling the gas turbine engine.
The turbine section of the engine includes an outer casing which circumscribes the working medium flow path. Arrays of stator vanes extend inwardly from the outer casing across the working medium flow path for directing the working medium gases. Arrays of rotor blades attached to a rotor assembly extend outwardly across the working medium flow path for extracting work from the working medium gases as the gases are flowed along the working medium flow path. An outer air seal at each array of rotor blades is supported and positioned from the outer casing. The outer air seal circumscribes the tips of rotor blades and is closely spaced to the rotor blades to confine the working medium gases to the working medium flow path as the gases pass through the array of rotor blades.
Ideally, there would be no clearance gap between the rotor blades and the outer air seal. However, the tips of the rotor blades move outwardly toward the outer air seal in response to heating by the working medium gases and rotational forces as the blades are driven about their axis of rotation by the hot, high pressure working medium gases. The outer casing which positions the outer air seal about the array of rotor blades is not subjected to rotational forces and is more remote from the working medium flow path than the rotor blades which are bathed in the hot gases. As a result, relative differences in growth occur between the rotor blades and the outer case. A clearance gap must be provided between the outer air seal positioned by the case to avoid a destructive interference between the blades and the seal during transient operating conditions. The clearance gap is desirably large enough to accommodate the maximum differences in thermal growth between the rotor assembly and the outer air seal and is small enough to minimize the adverse impact of working medium gases leaking between the tips of the rotor blades and the outer air seal
In modern engines, the outer case which positions the outer air seal is coolable to vary the diameter of the case and thus the radial gap between the array of rotor blades and the outer air seal. Examples of such constructions are shown in U.S. Pat. No. 4,019,320 issued to Redinger et al. entitled EXTERNAL GAS TURBINE ENGINE COOLING FOR CLEARANCE CONTROL and U.S. Pat. No. 4,247,248 issued to Chaplin et al. entitled OUTER AIRSEAL SUPPORT STRUCTURE FOR GAS TURBINE ENGINES. In these constructions, coolable rails extend circumferentially about the exterior of the outer casing.
Pressurized cooling air for this purpose is ducted rearwardly from the compression section or the fan section of the engine to a plurality of spray bars. The pressurized cooling air is impinged through the spray bars on the outer casing of the engine to change the temperature of the casing. The diameter of the case decreases forcing the case and the outer air seal positioned by the case to a smaller diameter. The tip clearance between the array of rotor blades and the outer air seal decreases and an increase turbine efficiency results.
Although an increase in turbine efficiency results in increased engine performance, the increase in performance is diminished by the use of cooling air. The gas turbine engine uses energy to pressurize the cooling air; energy that might otherwise be used for propulsion. Any reduction in the amount of cooling air needed to position the case reduces the performance penalty caused by the work of pressurization. In addition, it is desirable to increase the response time of the outer air seal to cooling of the case to enable the turbine to quickly reach the desired level of turbine efficiency.
Accordingly, scientists and engineers are working to design a coolable stator assembly for use in externally cooled turbine sections that improves the performance of the engine by decreasing the amount of cooling air required to achieve a given clearance and by decreasing the response time of the stator assembly to cooling to decrease the response time needed to achieve a given clearance between the blade tips and the outer air seal.